1. Field of the Invention
This invention relates to an amplifier using pulse-width modulation.
2. Description of the Prior Art
In general, amplifiers using pulse-width modulation enable efficient power amplification of signals. In such pulse-width modulation amplifiers, a high-frequency triangular signal (a carrier) is modulated with an input analog signal so that the input analog signal is converted into a corresponding pulse-width modulation signal. Then, the pulse-width modulation signal is subjected to power amplification. A low pass filter removes carrier components from the amplified pulse-width modulation signal, demodulating a signal which is equal to a resultant of power amplification of the input analog signal. The demodulated signal is fed to a load. The pulse-width modulation amplifiers are used in various devices such as audio amplifiers or switching power supplies.
Some prior-art pulse-width modulation amplifiers for audio use include a triangular signal oscillator, a voltage comparator, a power amplifier, and a low pass filter. The voltage comparator compares an input audio analog signal with a triangular signal (a carrier) outputted from the triangular signal oscillator, and thereby converts the input audio analog signal into a corresponding pulse-width modulation signal. The power amplifier subjects the pulse-width modulation signal to power amplification, outputting an amplified pulse-width modulation signal. The low pass filter removes carrier components from the amplified pulse-width modulation signal, demodulating a signal which is equal to a resultant of power amplification of the input audio analog signal. The demodulated signal is fed to a load such as a loudspeaker.
Such a prior-art pulse-width modulation amplifier has the following problem. The voltage of the output signal from the prior-art pulse-width modulation amplifier is substantially proportional to a power supply voltage fed to the power amplifier within the pulse-width modulation amplifier. Accordingly, in cases where a power supply voltage fed to the power amplifier has ripple noise of a certain level, the output signal from the prior-art pulse-width modulation amplifier tends to be significantly contaminated by ripple noise.